The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a PMOS transistor and a method for forming a dual gate of a semiconductor device using the same.
Polysilicon is generally used as a material for forming a gate of a semiconductor device. This is because the polysilicon meets physical properties required for the gate material such as high melting point, easiness of thin film formation and line pattern formation and formability of an even surface. Conventionally, for process simplification, the gate is formed of N-type doped polysilicon in both NMOS and PMOS transistors and the PMOS transistor is therefore formed with a buried channel. However, as a design rule is decreased more and more and high power and high speed operation are required, the PMOS transistor with the buried channel represents a limitation. In order to overcome the limitation, a dual gate process is widely used in recent, in which N-type doped polysilicon is used in an NMOS region and P-type doped polysilicon is used in a PMOS region.
By changing the PMOS transistor into a surface channel type, current on/off ratio and role off property of threshold voltage of the short channel are improved. It is necessary to use P+ doped polysilicon as the gate material instead of N+ doped polysilicon to form the PMOS transistor in the surface channel structure.
Conventionally, in order to form the gate of the PMOS transistor, a polysilicon layer is, after deposition of the polysilicon layer, doped with P-type impurities by ion implantation or plasma doping. However, the method of doping the polysilicon layer by the ion implantation takes more than 30 minutes per a wafer for implanting a large amount of P-type dopant and has a problem of low mass productivity. In the case of the plasma doping, although the mass productivity is ensured, a large amount of P-type dopant piled on the polysilicon layer together with being diffused into the polysilicon layer. Boron (B) not implanted into the polysilicon layer but piled on the polysilicon layer promotes growth of an oxide layer to form a thin oxide layer between the polysilicon layer and a tungsten layer.
Such oxide layer formed between the polysilicon layer and the tungsten layer causes ring oscillator delay. Also, in the case of the plasma doping, it is necessary to purchase a new plasma doping equipment of high price.